Some optical disk recorders employ constant lineal speed control, i.e., as the laser beam scans radially more outward tracks, the rotational speed of the disk is reduced to maintain a constant lineal track scanning speed. Such recorders are often employed for audio and video recording. On the other hand, data recording devices, such as used with computers and the like, employ a constant rotational speed. It is to be appreciated that at a constant rotational speed a signal of a given duration is recorded as a longer mark in a radially-outward track on the disk than when recorded on one of the radially-innermore tracks. In pulse-position-modulation (PPM) recording, the change in recorded mark lengths create different recording tolerances at different radii of the disk. Different recording formats also result in different recording tolerances. Since the scanning time for the different length marks is relatively constant, slight variations in speed and responsiveness of the recording media, particularly magnetooptic media, can be tolerated. However, when an encoding method known as pulse-width modulation (PWM) is employed, then the tolerances required for successful readback of the recorded information are reduced. PWM provides for higher linear recording densities than PPM. In pulse-width modulation the duration of a pulse recorded on a record track is varied for indicating different informational values. For example, a relatively short duration pulse would represent binary zero, a slightly longer pulse represents unity, a yet longer pulse representing two, and so forth. A single recorded pulse can represent a number up to modulo 10 or 16. Pulse-width modulation greatly enhances the data storing capability of a record media. It is to be appreciated that this greater storage capability comes at a severe price in that distinguishing between various pulse lengths is usually a difficult readback operation, particularly for interchangeable media. That is, one recorder may record pulses which tend to be long while another recorder records pulses which tend to be short. Somewhere in between the short and long pulses a region of ambiguity exists wherein the exact informational content of a given pulse may not be reliably and readily determined even by sophisticated and complex readback circuits. Accordingly, it is desired to provide a uniformity in recording for facilitating pulse-width modulation of information-bearing signals onto optical media such that not only is signal readback facilitated, but that interchange of media among a plurality of recorders is made more reliable.
Optical recording has used constant intensity laser beams for creating recorded pulses on optical media. It is also well known that pulsed or "serrated" writing signals can also be applied. That is, a series of short duration pulses effectively record a single long pulse on the record medium. Thermal diffusion of the heat induced into the recording layer by the recording laser beam also causes distortion of the recorded pulse in optical recording. It is desired to minimize the negative effects of such thermal diffusions by carefully calibrating the laser write pulses to provide a uniformity of recording among a plurality of optical disk recorders.